1. Line jelly-roll pan with foil. Place fish on foil; sprinkle each cavity with 1/2 teaspoon of the rosemary and sprinkle lightly with salt and pepper. Place several lemon slices in each cavity. Rub remaining 1-1/2 teaspoons rosemary over fish. Cover; refrigerate up to 8 hours.
2. Heat oven to 450 degree F. Remove fish from refrigerator; let stand 15 minutes. Drizzle with olive oil. Roast 30 minutes or until fish flakes easily with a fork. Transfer to serving platter; garnish with rosemary sprigs and lemon slices, if desired.Make Green Sauce::
3. Meanwhile, chop parsley and capers in food processor. Add mustard, the 1/4 teaspoon salt, and the 1/8 teaspoon pepper; pulse. With machine on, gradually add oil through feed tube until just blended. Serve with fish. Makes 12 servings.